NETMG7

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What is End-to-end QoS?

It is the ability of the network to deliver service required by specific network traffic from one end of the network to another.

QoS service model includes...

Best-Effort Service

Integrated Services(IS)

Differentiated Services(DS).

Best-effort service is suitable for...

A wide range of networked applications such as general file transfers or e-mail.

However, best effort service is not suitable for real-time applications such as VoIP and VOD since performance outcomes are not reliable.

What are the key building blocks of the Integrated Service?

Resource reservation and admission control are key building blocks of the service.

The IS model requires routers to be able to...

Reserve resources, in order to provide special QoS for specific user packet streams, or "flows". This in turn requires flow-specific state to be maintained in the routers.

What is defined as a distinguishable stream of related datagrams that results from a single user activity and requires the same QoS.

A "flow"

What are the limitations and security issues of the IS (Integrated Services) model?

Need to maintain individual end-to-end flow and path state at each intermediate hop.

Ability to reserve resources

Differentiated services...

Enhancements to the Internet Protocol are intended to enable scalable service discrimination in the Internet without the need for perflow state and signalling at every hop

What is DS behavior aggregate (BA)?

A collection of packets with the same DSCP value crossing a link in a particular direction.

What does DSCP stands for?

The Differentiated Services Codepoint(DSCP) is a value which is encoded in the DSField.

Each node in the network uses the QoS specification to...

To classify, mark, shape, and police traffic, and to perform intelligent queuing.

What is a PHB?

A per-hop behavior (PHB) is a description of the externally observable forwarding behavior of a DS node applied to a particular DS behavior aggregate.

What is the means by which a node allocates resources to behavior aggregates?

The PHB

Limitations of the DS model:...

• The lack of end-to-end bandwidth reservations mean that guarantees of services can be impaired by network nodes that do not implement the PHBsappropriately.

• The lack of a per flow call admission control makes it possible for applications of the same class to congest each other. For example, if only enough bandwidth for 10 voice calls exists and an 11thcall is permitted, all 11 calls suffer call-quality deterioration.

The Class-Selector PHB...

Preserves backward-compatibility with any IP precedence scheme currently in use on the network.

Assured Forwarding PHB...

Provides four classes, each with three drop precedence levels. Assured Forwarding (AF) PHB defines a method by which BAs can be given different forwarding assurances.

In instances of network traffic congestion, if packets in a particular AF class need to be dropped, packets with a higher dp value will be dropped...

First

Expedited Forwarding PHB...

Provides a single DSCP value (101110) that gives marked packets the highest level of service from the network.